The invention relates to an improvement of microwave integrated circuits, and more particularly to an improvement in a bonding structure of dielectric substrates of impedance matching circuits on a packaging substrate for microwave integrated circuits.
The microwave integrated circuits are provided with not only a semiconductor chip but impedance matching circuits on a packaging substrate as illustrated in FIGS. 1A and 1B. The impedance matching circuits are provided on dielectric substrates such as alumina substrates. A semiconductor chip 34 is provided on a center area of a packaging substrate 31. A pair of first and second dielectric substrates 35 and 36 are provided on the packaging substrate 31 at both sides of the semiconductor chip 34 or input and output sides respectively. The first and second dielectric substrates 35 and 36 are provided with first and second impedance matching circuits thereon. The first and second impedance matching circuits comprise first and second metallization patterns 45 and 46 formed on top surfaces of the first and second dielectric substrates 35 and 36. Each of the first and second metallization patterns 45 and 46 on the first and second dielectric substrates 35 and 36 is electrically connected through bonding wires to wire bonding pads 38 provided on the semiconductor chip 34. Each of the first and second dielectric substrates 35 and 36 has a bottom surface which entire area is metallized or covered with a metallized surface 47 as illustrated FIG. 1C. A soldering agent 37 is applied on the bottom metallizied surface 47 of each of the first and second dielectric substrates 35 and 36. The first and second dielectric substrates 35 and 36 are bonded through the soldering agent 37 on the packaging substrate 31.
As described above, in the conventional dielectric substrates for the impedance matching circuits, the top surface of the dielectric substrate is selectively metallized to form metallization patterns or microstrip lines that constitute distribution constant impedance matching circuits. By contrast, the bottom surface of the dielectric substrate is subjected to a full area metallization or entirely covered with a metalilized surface 47 which is bonded through the soldering agent on the surface of the packaging substrate. Namely, the entire bottom surface of the dielectric substrate is securely bonded on the surface of the packaging substrate.
As is well known, the packaging substrate 31 and the first and second dielectric substrates 35 and 36 receive repeatedly a temperature variation by a heat generated in driving the microwave integrated circuits. The packaging substrate 31 has a different coefficient of thermal expansion from that of the first an second dielectric substrates 35 and 36, for that reason the repeated temperature variations in driving the microwave integarted circuits may cause a considerable internal stress between the packaging substrate 31 and the first and second dielectric substrates 35 and 36. The packaging substrate 31 and the dielectric substrate receive such considerable internal stress whenever receipt of any temperature variations due to a heat generation in driving the microwave integrated circuits. The internal stresses generated every times of receipt of the temperature variations are accumulated both in the packaging substrate 31 and in the first and second dielectric substrates 35 and 36. The accumulation in the packaging substrate 31 and the first and second dielectric substrates 35 and 36 of the internal stresses generated whenever receipt of any temperature variations may cause the packaging substrate to be arched and further may results in an increase of a possibility of generation of cracking in the dielectric substrates or a ceramic substrate at a lead portion. The generation of cracking in the dielectric substrate or in the ceramic substrate due to the accumulation of the internal stresses in the packaging substrate and the dielectric substrate may result in a generation of any leakage current or a difficulty in an alignment at a necessary accuracy of the dielectric substrates including the impedance matching circuits to the packaging substrate. This results in a lowering of reliability of the microwave integrated circuits.
From the above, it could be understood that it is require to develop a novel bonding structure between dielectric substrates including impedance matching circuits and the packaging substrate.